Prostaglandins, particularly prostaglandin E2 (PGE2), are potent regulators of bone resorption and formation. The identification of four different receptor types for PGE2 has made it possible to examine the pathways for different PGE2 effects on bone. Since endogenous PGE2 plays a critical role in skeletal responses to fracture, inflammation, mechanical loading, and possibly hormone deficiency, a definition of the specific roles of prostaglandin receptors is essential to fully understand skeletal physiology and pathophysiology and may lead to new therapies for bone loss and fracture healing. Our studies will focus on the role of EP2 and EP4 receptors (EP2R and EP4R). Activation of either receptor increases cAMP, but there are different responses to specific EP2R and EP4R agonists. Our current hypotheses concerning the roles of EP2R and EP4R in bone are as follows: (1) EP4R expression in osteoblasts is necessary for maximal support of osteoclast formation and bone resorption in response to PGE2; (2) EP2R in osteoblasts facilitates these responses, but an additional effect of EP2R in hematopoietic cells, possibly T-cells, can further enhance osteoclast formation; (3) Both EP2R and EP4R can mediate PGE2 stimulation of bone formation. To test these hypotheses, we have developed three Specific Aims. The first Specific Aim will examine the individual effects of deletion of EP2R or EP4R using both in vitro and in vivo models. To obtain viable mice with EP4R deletion (-/-), the deletion will be targeted to cells of the osteoblast lineage using Cre-lox methodology and two collagen promoters, Col 3.6 and Col 2.3, which are activated at different stages of osteoblast differentiation. The second Specific Aim will examine double deletion of EP2R and targeted EP4R, which should abrogate the cAMP response to PGE2, as well as specific EP2R and EP4R agonists in osteoblastic cells. The bones from both single and double knockout mice will be studied in cell and organ culture, and in vivo bone turnover in response to specific perturbations will be examined. In the third aim, the specific cells of the hematopoietic lineage involved in the EP2R enhancement of osteoclast formation will be determined using wild-type spleen cells and immunoseparation of the cell types. In addition, signal transduction pathways for enhanced osteoclastogenesis will be examined.